Exhausted doesn’t mean anything, he was going Plus Ultra to surpass his limits. He noted that he was going to run out of energy, not that he was out of it fully or that he would conserve it. If anything, that should incentivize him to use it as hard as he can to get to Aizawa, which is what he tried to do.
And we’re saying he hit Deku comparably, and not a love tap like you’re implying. He did not go easy on Deku, unless you think Shigaraki is an idiot who doesn’t go all out when he needs to at the most crucial moment of his existence, when it has been established he loves going all out.
Plus Ultra is the MHA verse-specific thing where a character goes beyond their limits and becomes stronger or faster than before through sheer willpower. It is an observable phenomenon that has occurred several times, and is listed as “Awakened Power” on some of try profiles like Shigaraki, Deku and All Might.